The invention relates to a thermal head for use in thermal printers, and more particularly, to a thermal head capable of preventing degradation in print quality, caused by adherence of dirt or the like.
Generally, a thermal head as a recording head mounted on a thermal printer or the like comprises a plurality of heating elements composed of a heating resistor and aligned in a row on a substrate, and the heating elements are selectively energized according to printing information to generate heat, thereby melting ink on an ink ribbon to heat transfer the same to regular paper, paper for OHP (overhead projector), or the like, or to cause a thermal recording paper to take color, so that printing on recording media of various kinds is performed.
With such conventional thermal head, it is general as shown in FIG. 3 that a heat reserving layer 12 is formed on an upper surface of a radiating substrate 11 and a projection 12b is formed on an upper surface of the heat reserving layer 12 and near one end 11a, which constitutes a right side end of the substrate 11, to project a predetermined height.
Also, a heating resistor 13 is laminatingly formed on the upper surface of the heat reserving layer 12, and a common electrode 14 and an individual electrode 15 are formed on the left and right of the heating register 13 to supply electric power energy to the heating resistor 13.
A plurality of heating elements 13a are aligned in a dot-shaped manner and formed in a location between the common electrode 14 and the individual electrode 15 of the heating resistor 13.
Also, protective layers (not shown) are laminatingly formed on upper surfaces of the heating elements 13a, the common electrode 14 and the individual electrode 15 to prevent oxidation and abrasion of the heating elements 13a, and the respective electrodes 14, 15.
Also, as shown in FIG. 4, a driver IC 16 is arranged on a left side of the projection 12b in the figure and near the other end 11b of the substrate 11 to be connected to the common electrode 14 and the individual electrode 15.
Also, a terminal portion 17 formed from a FPC (flexible substrate) or the like is taken out from the other end 11b of the substrate 11.
With such conventional thermal head, the substrate 11 is mounted on a head mount (not shown) to be mounted on a thermal printer for printing, at which the head mount is turned to bring the thermal head into pressure contact with a platen (not shown), whereby the heating elements 13a can be brought into pressure contact with, for example, an ink ribbon 18.
In the case where a printer mounting thereon the conventional thermal head described above is a thermal transfer printer, the thermal head 11 is lowered to bring the heating elements 13a into pressure contact with the ink ribbon 18 and to move the thermal head 11 in a direction indicated by an arrow A.
Then, the heating elements 13a is caused on the basis of printing information to selectively generate heat to heat the ink ribbon 18, whereby ink on the ink ribbon 18 is transferred to a recording sheet 19 to afford printing characters, images or the like on the recording sheet 19.
Also, with a thermal transfer printer of line type, printing on the recording sheet 19 can be performed while moving the ink ribbon 18 and the recording sheet 19 in a direction indicated by an arrow B without moving the thermal head.
With conventional thermal heads, however, recesses 12c, 12d are produced on the surface of the heat reserving layer 12 and on right and left feet of the projection 12b of a predetermined height in the figure.
Therefore, when ink in the ink ribbon 18 is transferred to the recording sheet 19 placed on a platen (not shown) for printing while the thermal head 11 with its head lowered is moved in the direction of the arrow A, it is feared that foreign matters, such as fine dirt or the like, attaching to the ink ribbon 18 accumulate in the recess 12c on a left side of the heating elements 13a in the figure and upstream of the projection 12b in the direction of movement indicated by the arrow A.
When foreign matters such as dirt or the like accumulate in the recess 12c on the left side of the heating elements 13a in the figure as described above, it is feared that striped white lines or the like are generated on a picture image printed on the recording sheet 19 under the influence of dirt or the like to cause degradation in quality of printing.
Also, in the case where printing is performed while the ink ribbon 18 and the recording sheet 19 are moved in the direction of the arrow B and without moving the thermal head 11, it is feared that foreign matters such as dirt or the like accumulate in the recess 12c on the left side of the heating elements 13a in the figure.
The invention has been thought of in view of the above problem, and has its object to provide a thermal head capable of performing printing of high quality by preventing foreign matters such as dirt or the like at the time of printing to accumulate in a location where a heat reserving layer is formed.
The invention provides, as first solving measures for solving the above problem, a thermal head comprising a heat reserving layer formed on a surface of a substrate, a plurality of heating elements formed on an upper surface of the heat reserving layer, an individual electrode and a common electrode for supplying electricity to the heating elements, and a protective layer covering at least upper surfaces of the heating elements, the individual electrode and the common electrode, and wherein the heat reserving layer comprise a projection formed by partially projecting a surface of the layer, the heating elements being provided on a surface of the projection, and the projection is shaped in cross section in a direction perpendicular to a direction of arrangement of the heating elements to form an inclined surface on one surface side thereof, the other surface side thereof being formed to be flat in substantially the same height as that of a top of the projection.
Also, as second solving measures for solving the above problem, a height of the projection from the one surface side is 5 to 50 xcexcm.
Also, as third solving measures for solving the above problem, the common electrode is formed on the one surface side and the individual electrode is formed on the other surface side.